1. Field of the Invention
The present invention relates to a Czochralski single crystal pulling device and to a heat shield member that is used therefor.
2. Description of the Related Art
The Czochralski method is a method conventionally known for growing a single crystal silicon. The Czochralski method involves melting a raw material in a crucible, immersing a seed crystal in the melt thereof, and then, while rotating the seed crystal and the crucible, slowly pulling the seed crystal up from the melt to make a single crystal silicon grow thereunder.
The quality of a single crystal that is grown by the Czochralski method is generally known to be dependent on the temperature during the growing process. Radiant heat from the melt in the crucible is given as a factor having a major effect on the temperature of the crystal. In order to control it, a heat shielding member made of graphite or the like is normally placed around the crystal being grown in the Czochralski single crystal pulling device (for example, refer to Japanese Unexamined Patent Application First Publication No. H05-294784 (pages 2 to 4) and Japanese Unexamined Patent Application First Publication No. H11-290792 (page 4)).
Also, in the aforementioned single crystal pulling device, an optical instrument is provided for controlling the pull-up speed by measuring in a non-contact way the diameter of the crystal being grown. Therefore, a notch portion (including a slit) is often provided in the heat shielding member in order to ensure the visual field.
In the aforementioned single crystal pulling device, the temperature in the chamber is maintained at not less than 1,000° C. during the pulling up of a single crystal. On the other hand, after completion of the pulling up of a single crystal, the temperature in the chamber is reduced to room temperature, and in this state the single crystal raw material used for the next pulling is supplied to the crucible. That is, an extremely large temperature difference occurs in the chamber between supplying of the raw material and pulling up of the single crystal, and this repeatedly occurs during the manufacturing process of a single crystal. Also, a portion, which faces the melt, is provided on the bottom end portion of the above-mentioned heat shielding member. A temperature difference of 100° C. to 200° C. occurs between the center portion and the peripheral portion of that portion due to the effects of inert gas that flows along the surface thereof and the relationship between the arrangement of the crucible and the heater.
For this reason, in the above-mentioned heat shielding member, large stresses may occur during use due to the effects of the aforementioned severe temperature changes that occur in the chamber and the severe temperature differences that occur in the material, as well as the residual stress in the material and the thermal expansion differential between a covering material for degradation prevention and the inner material or the like. Above all, stress tends to concentrate in the vicinity of a notch portion that is provided for the optical instrument, so that as material quality degradation progresses with repeated use of the heat shielding member, there is a possibility of failure occurring during use. In such a situation, fragments or the like of the heat shielding member would become mixed with the melt in the crucible, rendering the melt unusable, and may become a cause for lowering the yield. In order to avoid this, it is conceivable to replace the heat shielding member before material quality degradation advances. However, since the heat shielding member itself is a comparatively expensive component, frequently replacing it would lead to a cost increase.
The present invention was achieved in view of the aforementioned circumstances, and has as its object to provide a heat shielding member with excellent durability that can relieve thermal stress that occurs in the material, and a single crystal pulling device that uses the heat shielding member.